It is known to apply a vacuum to glass canning jars. Such jars are sold under the registered trademarks Mason.RTM., Kerr.RTM., and Ball.RTM.. For the purpose of this invention, these jars may be described both in construction and operation.
As constructed, these jars have a lower seal rim defined below their open top. It is known in the prior art that this lower rim may be a location of a vacuum seal for drawing a vacuum typically when the jar is filled with food contents to be preserved.
These jars have an open cylindrical top protruding above the seal rim. The top of the jar terminates at an upper sealing surface at the top of the cylindrical top. The cylindrical surface is threaded on the exterior surface with a male thread conformed to standard dimension shared by all jar brands.
When the jar is originally sealed, a flexible lid, typically of metal, is placed over the top of the jar. This flexible lid has a peripheral female flange with a contained rubber gasket. When the jar is initially sealed, the flexible lid at the rubber gasket is place over the top of the jar. Presuming that the flexible lid is in place, when a vacuum is drawn to the interior of the jar, the lid seals at the gasket to the seal surface at the cylindrical top of the jar and remains in place. After a vacuum is established in the interior of the jar, a threaded top compressing the periphery of the metallic lid rim to the top of the jar is threaded over the jar at the thread. As a result, the flexible lid is held firmly in place and the vacuum original obtained in the interior of the jar preserved.
In a normal hot canning operation, the jar is filled with the contents to be preserved and the flexible lid is placed over the top of the can. The lid sealed jars are then heated until substantially the entire volume between the jar contents and the sealing lid is filled with water vapor. Thereafter, the jars are cooled. The water vapor filling the volume under the lid condenses and a "vacuum" is drawn on the contents of the jar. To assure maintenance of the vacuum drawn within the jar, a threaded top having a compressing inner extending annulus is threaded to the jar and utilized to compress the flexible lid at the peripheral gasket to the jar top at the upwardly exposed sealing surface.
It is sometimes desirable to draw a vacuum without the heating step described above. In the prior art, a flexible sealing lid was placed over the jar and its contents. A generally concave body with a vacuum attachment (hereafter referred to as a "bell") was fitted over the top of the jar sealed by the flexible lid. A seal was effected between the sealing rim and the sides of the bell by a gasket. The bell was spaced from the sealing lid by three pressure points, these points being spaced at 120.degree. intervals around the side of the lid bearing directly on the lid at the sealing rubber gasket of the lid. Vacuum was drawn interior of the bell. The lid flexed or bent upwardly between the pressure points from the bell on the lid responsive the to pressure interior of the jar and the lack of pressure in the bell. A vacuum was drawn on the interior of the jar.